Neither LL core nor AEC has monster stats for a siren!
Let me fix that:
No. Enc.: 2-5 Alignment: Chaotic Movement: (walking) 120' (40') / (swimming) 120' (40') Armor Class: 6 Hit Dice: 3 + 2 Attacks: 2 (claws) or special Damage: 1d4/1d4 or Charm Save: F3 Morale: 8 Hoard Class: VI XP: 100
Although appearing as a comely maidens in diaphanous clothing, Sirens are really disgraced denizens of the sea who have been banished to rocky islands for some transgression unknown to land dwellers.
Because of their remote habitats, sirens are rarely a problem for dungeoneers. However, experienced sailors take special care to avoid areas known to be frequented by these creatures.
The primary attack of sirens is their beguiling song. All within 1 mile (or those occupying the same standard sea hex) of a singing siren must save vs. spells or become charmed (as the 1st level Magic-User spell Charm Person).
Note that this spell-like effect targets an area and will affect all who have not taken precautions to stop the creature's sound from reaching his or her ears. The song effects all humanoids of 6 HD or fewer.
Those so ensorcelled will sail or, if unable to control their craft, swimming toward the closest island with sirens. Armor and weapons may likely be discarded by the victim to better prepare for a long swim. However, normal drowning rules apply (q.v. page 46 LL core).
Any charmed person arriving within reach of the siren will be attacked by the sirens, who will attempt to eat their prey.